Asterism Versus Constellation: Which Is Which?
Most people can probably locate the Big Dipper, and they probaby believe that it is one of the better-known constellations. However, the fact is the Big Dipper is not a constellation after all; it is an asterism. So, what is an asterism? What is the difference between constellations and asterisms?
Trapezoidal Asterism in Octans Constellation
To answer these two questions, we need to first define the terms asterism and constellation. Let's examine information from the International Astronomical Union (IAU) to help us in our defining process.
What is an asterism? According to IAU, “Asterisms are patterns or shapes of stars that are not related to the known constellations but nonetheless are widely recognized by laypeople or in the amateur astronomy community." (1) As IAU does not officially recognize obsolete or informal constellations, asterism is the more commonly used terminology.
Their patterns may include parts of one or more known constellations, or they may be comprised of several recognizable stars like Vega, or Deneb. Additionally, unlike constellations, which have recognizable boundaries, asterisms may or may not have a defined boundary, and can be a random pattern.
What is a constellation? Constellations are groups of stars with IAU-defined boundaries; currently the IAU recognizes 88 constellations, with boundaries defined in 1930 by Eugene Delporte.